The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)2(dtb-bpy)]+ Enabling Access to Energy Demanding Organic Substrates

Article abstract of DOI:10.1021/jacs.9b07370

We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using the [Ir(ppy)₂(dtb-bpy)]⁺ (1⁺) photocatalyst. The immediate products of photoinitiated electron transfer (PET) between 1⁺ and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (2). Formation of 2 occurs via reduction and semisaturation of the ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor, confirmed by nuclear magnetic resonance, mass spectrometry, and deuterium labeling experiments. Steady-state and time-resolved luminescence and absorption studies reveal that upon irradiation, 2 undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative acceptor (N-(diphenylmethylene)-1-phenylmethanamine; S). Turnover of this new photocatalytic cycle occurs along with the reformation of 1⁺. We rationalize our observations by proposing the first example of a mechanistic pathway where two distinct yet interconnected photoredox cycles provide access to an extended reduction potential window capable of engaging a wide range of energy demanding and synthetically relevant organic substrates including aryl halides.

Full text of DOI:10.1021/jacs.9b07370

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Article
2
The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)(dtb-
+
bpy)] Enabling Access to Energy Demanding Organic Substrates
Timothy U. Connell, Catherine L Fraser, Milena L Czyz, Zoe M Smith, David J Hayne,
Egan H. Doeven, Johnny Agugiaro, David J. D. Wilson, Jacqui Louise Adcock, Andrew
D Scully, Daniel E. Gomez, Neil W. Barnett, Anastasios Polyzos, and Paul S Francis
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07370 • Publication Date (Web): 23 Sep 2019
Downloaded from pubs.acs.org on September 23, 2019
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The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)
Demanding Organic Substrates
2
(dtb-bpy)] Enabling Access to Energy
a,
b
c
b
b
Timothy U. Connell, * Catherine L. Fraser, Milena L. Czyz, Zoe M. Smith, David J. Hayne, Egan H.
d
e
e
b
f
a
Doeven, Johnny Agugiaro, David J. D. Wilson, Jacqui L. Adcock, Andrew D. Scully, Daniel E. Gómez,
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Neil W. Barnett, Anastasios Polyzos, * Paul S. Francis *
aSchool of Science, RMIT University, Melbourne, VIC 3000, Australia
bSchool of Life and Environmental Sciences, Deakin University, Geelong VIC 3220, Australia
cSchool of Chemistry, The University of Melbourne, Parkville, VIC 3010, Australia
dCentre for Regional and Rural Futures, Deakin University, Geelong VIC 3220, Australia
eDepartment of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne,
VIC 3086, Australia
fCSIRO Manufacturing, Clayton, VIC 3168, Australia
*
Corresponding email addresses; tim.connell@rmit.edu.au (T. U. Connell), anastasios.polyzos@unimelb.edu.au
(
A. Polzyos), paul.francis@deakin.edu.au (P. S. Francis)
ABSTRACT
We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using
+
+
the [Ir(ppy)
2
(dtb-bpy)] (1 ) photocatalyst. The immediate products of photoinitiated electron transfer
+
(PET) between 1 and triethylamine (TEA) undergo subsequent reactions to generate a previously
unknown, highly reducing species (2). Formation of 2 occurs via reduction and semi-saturation of the
ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor,
confirmed by nuclear magnetic resonance, mass spectrometry and deuterium labelling experiments.
Steady-state and time-resolved luminescence and absorption studies reveal that upon irradiation, 2
undergoes electron transfer or proton-coupled electron transfer (PCET) with a representative
acceptor (N-(diphenylmethylene)-1-phenylmethanamine; S). Turnover of this new photocatalytic
+
cycle occurs along with the reformation of 1 . We rationalize our observations by proposing the first
example of a mechanistic pathway where two distinct yet interconnected photoredox cycles provide
access to an extended reduction potential window capable of engaging a wide range of energy
demanding and synthetically relevant organic substrates including aryl halides.
INTRODUCTION
Transition metal complexes are central to recent strategies in chemical synthesis due to the breadth
and ease of small molecule activation afforded under visible light irradiation.1 The first examples
and much of the recent resurgence was enabled by ruthenium(II) and iridium(III) complexes (Scheme
a) possessing long-lived triplet excited states and tunable redox properties that allow the catalyst to
be closely matched to substrate energy requirements. In a representative photoredox cycle (Scheme
-9
10
1
11
+
+
1b) using [Ir(ppy)
2
(dtb-bpy)] (1 ) as the catalyst, the reaction proceeds following absorption of visible
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light to generate the electronically excited state 1 * which is then reduced via electron transfer from
a sacrificial donor (such as triethylamine (TEA)). This intermediate ([Ir(ppy) (dtb-bpy )] , 1 ) reduces
the substrate (S), thereby completing the cycle to re-generate the catalyst. An analogous cycle can be
•−
0
0
2
drawn for reactions in which the electronically excited state of the complex is first oxidized.1
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Scheme 1. Transition metal photoredox catalysis. (a) Examples of common photoredox catalysts and their
relevant reduction potentials (quoted vs SCE; see ref. 1). Excited state potentials of 1 were determined using
0-0 values estimated from the low-temperature luminescence spectrum of 1 ; see Supporting Information. (b)
(dtb-bpy)] , 1 ; (ii) reductive
quenching of the electronically excited state 1 * by a sacrificial electron donor (TEA); and (iii) reduction of the
substrate, S. (c) Examples of energy demanding substrates previously reported to engage in this mechanistic
a
+
+
E
+
+
Representative photoredox cycle involving: (i) photoexcitation of [Ir(ppy)
2
+
cycle.
The scope of any photocatalyst operating through these mechanisms is, therefore, fundamentally
limited by the redox potentials of its ground and excited electronic states. Thus there is an ongoing
challenge in organic chemistry to advance photocatalytic methodology beyond the inherent redox
12
2+
properties of the catalysts. An elegant approach to extend the reduction potential of [Ru(bpy)
through ‘sensitization-initiated electron transfer’ (senI-ET) enabled access to new substrates.1
3
]
3-14
Energy is transferred from the metal catalyst excited state to an intermediary species, such as pyrene,
which then undergoes electron transfer with a substrate, although the exact nature of this process is
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vigorously debated.15-16 Reported systems where simple electron transfer between the catalyst and
substrate proceeds efficiently despite being endergonic based on redox potentials are also poorly
understood. In substrates where hydrogen-bonding opportunities exist, this unexpected reactivity is
often reconciled by concerted proton-coupled electron transfer (PCET).1 The thermochemistry of
this hydrogen atom transfer process is governed by the bond dissociation free energy (BDFE),
1
2
7-18
calculated using the pK
a
and reduction potential. In a single reagent, these properties are often
1
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opposed, and it is not possible to alter the BDFE to match the substrate’s energy requirements.1
9
Multisite PCET separates pK and reduction potential across two species, providing an effective BDFE
that is now manipulatable. For example, an iridium photocatalyst acting as a reductant combined with
a
red
a Brønsted acid provided access to aldehydes, ketones and imines with reduction potentials (E
p
)
extending beyond −2.0 V vs SCE.20-21 Under conditions where hydrogen bonding is unlikely, such as
red
+ 22
the reduction of unactivated alkyl iodides (E
p
≤ −2.5 V vs SCE) using photoredox catalyst 1 , the
observed reactivity was tentatively ascribed to exciplex formation between the substrate and
sacrificial electron donor.
We recently reported a chemoselective method for the photoredox mediated transfer hydrogenation
23
of imines (Scheme 1b,c). Stern-Volmer quenching experiments revealed the excited state of the
+
catalyst 1 is quenched by TEA and transient absorption measurements confirmed this interaction
occurred via reductive quenching photoinitiated electron transfer (PET) to form the reduced complex
0
+
1
. The quenching of 1 * by TEA was previously demonstrated in photocatalytic water reduction
24-25
26-32
studies,
and included in the postulated mechanisms of numerous synthetic transformations.
0
+
Subsequent oxidation of 1 back to 1 was proposed to occur via electron transfer between the
transition metal complex and the imine substrate, despite a large difference between the formal
potential of 1 (E ′(1 /1 ) = −1.47 V vs SCE) and the peak potentials of the imines (e.g., E
23
0
0
+
0
red
p
= −2.18 V
vs SCE for N-(diphenylmethylene)-1-phenylmethanamine; Scheme 1c). Herein we reveal a heretofore
+
0
unknown mechanistic cycle of the iridium photoredox catalyst 1 , in which the 1 intermediate is
converted into a new complex characterized by a large negative shift in reduction potential. The
product of this transformation is shown to undergo a dynamic PET interaction with imine substrates
using both steady-state and time-resolved luminescence and absorption experiments. We propose a
mechanism comprised of two distinct photoredox cycles operating in tandem and apply this system
to engage energy demanding substrates. These findings have implications for the mechanistic
understanding of iridium photoredox catalysis, as well as the design of new transition metal catalysts
and synthetic methodology.
RESULTS AND DISCUSSION
0
The fate of 1 . Formation of a new iridium complex
During the course of investigating the spectroscopic behavior of the iridium catalyst in the presence
of electron donors, we noticed that prolonged irradiation of a de-oxygenated acetonitrile solution of
1+
(0.1 mM) and TEA (50 mM) using blue light (max = 454 nm) gave rise to a dramatic change in the
luminescence of the solution from yellow to green (Figure 1a). This change is characterized by a new
intense emission displaying a large hypsochromic spectral shift and a more structured spectral profile
having maxima at 489 nm and 510 nm (Figure 1a), more consistent with emission from the higher
3
3
energy triplet ligand-to-ligand and metal-to-ligand charge-transfer ( LLCT and MLCT(ppy)) states of
+
33-35
1
*.
The absence of longer-wavelength yellow emission suggests that the rapid relaxation to the
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lowest-energy excited state, MLCT(dtb-bpy), of 1 is inhibited in this new species. The new green
emission has a longer lifetime (τ = 2.22 μs, Figure S1) than that of 1 * (τ = 0.57 μs), indicating the
excited state is likely still of triplet character. Without irradiation, the intensity of the green emission
was found to diminish over time, which occurred much more rapidly if oxygen was introduced
Supporting Information; Figure S2). The lack of significant change in the absorption spectrum of the
solution after irradiation (Figure 1b), other than the loss of a shoulder at 300 nm, suggests the new
green emission originates from a triplet excited state of an iridium-based complex closely related to
, but possibly with a smaller contribution of MLCT(dtb-bpy) character as reflected by the decreased
+
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radiative decay rate and increased structure in the emission spectrum.
(
a)
(b)
(c)
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Figure 1. (a) Luminescence spectra of 1 (orange curve) and a solution of 1 (0.1 mM) and TEA (50 mM) measured
at successive 46 s intervals upon irradiation with blue light (green curves). Inset: photographs of the emission
colour before and after irradiation. (b) Absorption spectra of 1 and TEA before (orange curve) and after (green
curve) irradiation, and 1 (black curve) produced by bulk electrochemical reduction. Inset: spectra highlighting
+
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the MLCT absorption bands of 1 and 2. (c) Transient absorption spectra of 1 * (orange curve) and 2* prepared
in situ (green curve), recorded 0.05 μs after excitation by a pump laser pulse at 355 nm.
Some organic photoredox catalysts have been proposed to undergo consecutive photo-induced
36-42
electron transfer (conPET) events allowing access to new higher energy intermediates,
although a
43
recent investigation has questioned this mechanism. To explore the possibility that this emission
originated from 1 * formed via conPET, spectroscopic measurements were conducted on solutions of
produced by direct electrolytic reduction of 1 (Figure S3). Upon reduction, the color of the solution
changed from a weak yellow to an intense orange (Figure S4), characterized by intense absorption
bands at 500 and 530 nm (Figure 1b), and a weaker, broad absorption band extending beyond 800
nm. These bands were not observed in the absorption spectrum of 1 in the presence of TEA before
or after irradiation (Figure 1b). Importantly, no luminescence was detected from the solution of 10
produced electrolytically (Figure S5). Time-dependent density functional theory (TDDFT) calculations
on 1 and 1 (Figure S6) produced spectra displaying similar distributions to those observed
experimentally (Figure 1b), but with higher absolute energies. Moreover, the spectra from TDDFT
calculations for 1 , 1 , and 1 (Figures S6-S7, and Tables S1-S4; also see supporting information for
molecular orbital (MO) calculations) all showed significantly lower energy absorption bands than
those observed for 1 before or after prolonged irradiation in the presence of TEA (Figure 1b). These
1
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results provide compelling evidence that the green phosphorescent species is not the triplet excited
0
state 1 *, but rather a new iridium-based complex that will be designated as 2 in this work.
Elucidating the identity of iridium complex 2
0
Confident that the observed emission is not the result of the excited state 1 *, we further probed the
nature of this new iridium species. The transient absorption (TA) spectrum of the triplet excited state
of the new species, 2*, displays prominent absorption bands at 370 nm and 460 nm, a broad
absorption band at wavelengths >650 nm, and overlaid on these, the phosphorescence signal
centered at around 500 nm (Figure 1c). The equivalent rates of decay of the TA and phosphorescence
signals (Figure S8) means that the same triplet excited state is responsible for both the transient
absorption and luminescence signals. The TA spectrum of 2* closely resembles that of the triplet
excited state of 1 * (Figure 1c), apart from the obvious hypsochromic shift of the peak at 390 nm in
the spectrum 1 * which was previously assigned to absorption by the radical anion dtb-bpy . The
shift in this high energy TA band to 370 nm suggests the radical anion formed in the charge-transfer
excited state of 2* is instead localized on the ppy ligands, indicating that the formation of 2 involves
+
+
•− 23
44
a chemical transformation of the dtb-bpy ligand.
+
Electrospray ionization-mass spectrometry (ESI-MS) of 1 revealed a single peak (m/z 769) with a
diagnostic iridium isotope pattern (Figure 2a). In contrast, analysis of a solution containing 2 prepared
in situ displayed two major iridium-containing cations at m/z 773 and 872. Tandem MS/MS
experiments revealed that both peaks correspond to different ionization states of the same parent
complex (772 a.m.u.) enabling their assignment as [2+H] and [2+IC] , where IC is an iminium cation
formed following PET from TEA to 1 *; vide infra). High-resolution mass spectra support these peak
] fragment (m/z 501) appears in the MS/MS pathways of
, [2+H] and [2+IC] and is consistent with spectroscopic observations that suggest structural changes
to the ancillary ligand. The charge neutrality of the resulting metal complex, as evinced by competing
ionization pathways, also supports conversion of the neutral dtb-bpy into a new mono-anionic ligand.
+
+
+
(
+
assignments (Figure S9). The same [Ir(ppy)
2
+
+
+
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Figure 2. (a) Selected ESI mass spectra (top to bottom) of 1 and 2 prepared in situ, and MS/MS spectra of peaks
1
at m/z 769, m/z 773, and m/z 872. (b) Partial H NMR spectra (500 MHz, CD
3
CN, 298 K) of the aromatic hydrogens
+
adjacent to the C-Ir organometallic bond of the cyclometallating ligands, following cumulative irradiation of 1
+
and TEA. Orange highlighted regions are assigned to 1 and green highlighted regions to 2. Spectra following
+
irradiation have been scaled relative to 1 to account for loss of signal.
+
A solution comprising of 1 (5 mM) and TEA (50 mM) subjected to increasing irradiation times was
characterized by H NMR spectroscopy. A significant loss of signal intensity for peaks associated with
occurs over 20 minutes of irradiation, along with the evolution of a multitude of new peaks (Figures
S10-S11). The absence of peak broadening confirmed that the products are diamagnetic, and that no
radical-containing species are formed on a timescale measurable using NMR spectroscopy. Focusing
on the diagnostic region of the H spectrum at δ 5.80–6.30 ppm (Figure 2b), assigned to the aromatic
hydrogens adjacent to the organometallic C–Ir bond of the cyclometallating ligands, a single major
product is formed where the original signal in 1 (δ 6.28 ppm) gives rise to two new signals (δ 6.13 and
.20 ppm). The resulting nonequivalence of the two cyclometallating ligands reveals that the original
2v symmetry is lost and supports non-symmetric alteration of the ancillary ligand. Analysis of a
1
+
1
1
+
6
C
reaction mixture by high performance liquid chromatography (HPLC) confirmed the presence of
byproducts suggested by NMR (Figure S12). In addition to the major green emitting species there
appears to be alternative iridium complexes that exhibit similar absorption spectra to 1 and 2, in
+
addition to products derived from the radical oxidation products of TEA (Figure S13).
Attempts to isolate 2 from a preparative scale reaction yielded less convoluted NMR spectra (Figures
S14-S17). Removal of excess TEA, as well as its photoproducts, revealed two non-equivalent peaks (δ
.94 and 1.31 ppm) assigned to the tert-butyl groups of the ancillary ligand as well as several non-
0
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of nonequivalent cyclometallating ligands is further supported by the C spectrum; many of the C
signals assigned to the ppy ligands appear to split into two closely related peaks (Figure S16 and S17);
the signal at δ 132.5 ppm in 1 , assigned to the carbon adjacent to the organometallic bond, splits into
two nonequivalent peaks upon conversion to 2. Homo- and hetero-nuclear 2D experiments highlight
the differences between the two tert-butyl groups. The downfield H signal (δ 1.31 ppm) shows
+
1
+
heteronuclear HMBC correlations that are very similar to that of 1 (δ 30.4, 36.3, 164.5 ppm vs δ 30.4,
1
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36.4, 165.0 ppm, respectively) while the upfield peak (δ 0.94 ppm) substitutes its downfield
correlation for a significantly less deshielded carbon (δ 26.8, 32.9 and 40.0 ppm; Figure S18). This
1
carbon signal also shows a HSQC correlation to the new H multiplet at δ 1.69 ppm, revealing that the
2
3
1
previously quaternary sp carbon is now a tertiary sp center (Figure S19). This H multiplet also
displays COSY cross-peaks with both an aromatic peak (δ 7.63 ppm) and many of the other non-
aromatic hydrogen peaks that appear in the spectrum of 2 (Figure S20-S21). These correlations
strongly suggest dearomatization of the pyridine heterocycle of the ancillary ligand evidenced by the
appearance of methylene (CH ) signals. Saturation of the tert-butyl functionalized carbon in one
2
pyridine ring introduces at least one stereocenter to the ancillary ligand and the resulting
+
diastereomers (1 is a racemic mixture; Δ/Λ isomerism) likely account for the multiple 2D
heteronuclear correlations of the tert-butyl carbons (Figures S18-S19).
+
Combining all the spectroscopic and structural evidence, the reaction 1 →2 is assigned to a loss of
+*
aromaticity and semi-saturation of the dtb-bpy ligand following the initial PET between 1 and TEA
(
Scheme 2a). Select examples of bipyridine dearomatization in coordination complexes exist in the
45-49
literature and all feature deprotonation of the nitrogen atom.
Transformation of the ancillary
ligand is likely to proceed through several steps whilst introducing new stereocenters, and formation
of byproducts and diastereomers as observed by HPLC and NMR is not unexpected. Irrespective of
either a concerted or stepwise hydrogen atom transfer, TEA can act as a source of electrons, protons
or hydrogen atoms as required. The use of TEA as an electron donor for the photo-induced reduction
+
•+
of 1 * generates the radical cation TEA , which initiates either hydrogen atom transfer or
•
50
deprotonation, giving the strong reductant TEA (Scheme 2b). This neutral α-amino radical (indirectly
observed via electrochemiluminescence experiments, vide infra) may be responsible for further
electron transfer to give the corresponding iminium cation IC, which is observed by both H NMR and
1
ESI-MS. Coupling of electron transfer and proton transfer in photoredox catalysis is often invoked in
substrate transformations1 and a cyclometalated iridium hydride complex formed following PET
7-18
51
with triethanolamine proved capable of CO reduction. Deuterium labelling confirmed that TEA was
2
the hydrogen source in the formation of 2. Substitution with d15-TEA yielded a corresponding shift in
the mass of the two major species identified by ESI-MS, although peak position and isotope pattern
1
suggested deuteration was not quantitative (Figure S22). The diagnostic H NMR signals assigned to
the reduced heterocycle were absent when using d15-TEA although the evolution of peaks in the
aromatic region was similar (Figures S23-S24).
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+
Scheme 2. (a) Representation of the proposed dtb-bpy ancillary ligand transformation when 1 is irradiated in
the presence of TEA to form the new iridium complex 2. The generated stereocenters, leading to diastereomer
formation, are labelled with *. [H] denotes formal addition of a hydrogen atom that may occur via one of three
pathways; hydrogen atom abstraction, concerted proton and electron transfer, step-wise proton and electron
transfer. (b) Triethylamine (TEA) enables all of these proposed hydrogen addition pathways following its
•
+
oxidation to form the amine radical cation (TEA ).
Reductive quenchers capable of hydrogen atom transfer are necessary for the formation of 2.
Replacing TEA with closely related N,N-diisopropylethylamine and triisobutylamine resulted in the
same loss of emission from 1 and growth of green emission, ascribed to 2* (Figure S25). Substitution
for 1,4-diazabicyclo[2.2.2]octane (DABCO) showed no change in emission color although quenching of
the excited state 1 * was observed. The strained nitrogen bridgehead in DABCO hinders both
deprotonation and hydrogen abstraction therefore preventing formation of 2. The absorption bands
+
+
•+
0
characteristic of the reduced photocatalyst 1 were similarly not observed, likely due to rapid back-
•
+
52
electron transfer to the stable DABCO radical (which persists for several seconds in acetonitrile).
These findings indicated that an electron donor that rapidly decomposes after single-electron
oxidation to products not capable of proton or H-atom transfer should result in the accumulation of
, without generating 2. The organic-soluble tetra-N-butylammonium oxalate (TBAOX), with
0
53
1
and CO •
−
, should satisfy both of these criteria. To our delight, the
decomposition products of CO
2
2
+
0
irradiation of 1 and TBAOX produced significant concentrations of 1 , with no conversion to 2 (Figure
S26), thus providing clear evidence for the role of TEA in the formation of species 2 responsible for the
green emission.
Electrochemical properties and excited state potentials
+
Cyclic voltammetry of 1 (Figure 3b, orange curve) shows reversible one-electron processes assigned
to the oxidation of the iridium atom (E ′(1 /1 ) = 1.24 V vs SCE) and reduction of the dtb-bpy ligand
E ′(1 /1 ) = −1.47 V vs SCE). Additional one-electron processes are observed following the first
0
2+
+
0
+
0
(
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reduction (Figure 3b, dashed orange curve). Under cathodic potentials, 2 exhibits two distinct, quasi-
reversible reductions at E ′(2/2 ) = −1.97 V and E ′(2 /2 ) = −2.21 V vs SCE (Figure 3b, green curve).
The position of the first peak is similar to the second reduction peak of 1 (Figure 3b, dashed orange
curve). The oxidation of 2 is obscured by the large excess of TEA in solution, and so an alternative
0
−
0
−
2−
+
approach exploiting the electrochemiluminescence (ECL) of the two complexes was devised.
54
Essentially the reverse process to photoredox catalysis, ECL is the emission of light from chemical
reactions between species generated by an applied electrochemical potential.5 By positioning a
photomultiplier tube at the flat transparent base of the electrochemical cell, we detected
luminescence from a solution of 1 and TEA during a potential sweep that was sufficient to oxidize
both species (Figure 3c). In this case, TEA serves as a ‘co-reactant’, where its oxidation generates the
1
1
1
1
1
1
1
1
1
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2
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0
5-56
+
•
o
57
+
highly reducing intermediate TEA (E = −1.6 V vs SCE; Scheme 2b) capable of generating 1 * upon
2
+ 58-59
reaction with the oxidized 1 .
The onset voltage for ECL is in good agreement with the potential
of 1 determined by cyclic voltammetry (Figure 3b,c, orange curves). Applying the same potential
sweep with 2, the onset of ECL was observed at 0.88 V vs SCE. The resultant electrochemical potential
gap obtained for 2 (E = 2.85 V) compared to 1 (E = 2.71 V) is reasonable, considering the large
hypsochromic shift in emission (Figure 1a). Replacing the photomultiplier tube with a CCD
+
+
+
spectrometer enabled the collection of ECL emission spectra, which confirmed 1 * and 2* as the
source of emission in each reaction (Figure 3d).
Figure 3. Cyclic voltammograms of (a) N-(diphenylmethylene)-1-phenylmethanamine (S), (b) complexes 1+
orange curves) and 2 (green curve), prepared in situ. (c) The onset of ECL during a forward potential sweep for
complexes 1 (orange curve) and 2 (green curve). (d) ECL emission spectra upon application of 1.4 V vs SCE for
0 s, for complexes 1 (orange curve) and 2 (green curve). Inset: photographs of the luminescence from the
(
+
+
2
+
+
working electrode surface. The ECL spectrum of 1 was obtained from a solution containing 1 (0.2 mM) and TEA
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(
50 mM) prior to irradiation, and the ECL spectrum of 2 was obtained after irradiating the same solution for 20
minutes.
+
o
2+
+
o
+
0
Excited state potentials of 1 * (E (1 /1 *) = −1.15 (±0.05) V and E (1 */1 ) = 0.92 (±0.05) V vs SCE)
were estimated using the difference in energy between the zeroth vibrational levels of the ground and
excited states (E0-0), approximated from low-temperature emission spectra (Figure S27-S28). These
0
1
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0
+
values differ from commonly quoted excited state potentials of 1 that were estimated using emission
spectra measured at ambient temperature (see Supporting Information for full derivation and
comparison). In conjunction with the E0-0 of 2.58 eV, approximated from the low-temperature
emission of 2 (max = 481 nm; Figure S29), we estimate the potentials of the electronically excited 2*
at E (2 /2*) = −1.70 V and E (2*/2 ) = 0.61 V vs SCE. Moreover, the observation of ECL in these
experiments confirms that TEA is generated under the reaction conditions employed in photoredox
o
+
o
−
•
+
catalysis (where TEA is oxidized by 1 * rather than electrochemically).
The tandem photoredox catalysis mechanism
Following elucidation of the electrochemical and optical properties of 2, we explored the role of this
species in the photoredox mechanism of 1 . A flow system that rapidly circulated solution between a
photoreaction chamber and a fluorescence cuvette (Figure S3) was used to continuously monitor the
concentration of 1 (λabs = 530 nm) and 2 (λem = 465 nm) during irradiation of a deoxygenated solution
containing 1 (0.1 mM) and TEA (50 mM). Irradiation resulted in the rapid formation of 1 , followed
by a slow exponential decay on a timescale of minutes, with a concomitant increase in the
phosphorescence intensity from 2 (Figure 4a,b). The slight decrease in phosphorescence intensity at
longer times suggests that 2 may be susceptible to decomposition upon prolonged irradiation of the
reaction mixture. The maximum rate of formation of 2 exhibited a non-linear relationship with respect
to light intensity (Figures S30-S31), suggesting that more than one photon is required for this
transformation. Upon addition of the imine substrate (S), we observed a retardation of the growth of
+
0
+
0
0
absorption by 1 and the green emission from 2* (Figure 4a,b). Notably, the characteristic absorbance
of 1 persisted over the full experiment timescale and emission spectra collected at the conclusion of
the experiment revealed a significant concentration of 1 remained in the reaction mixture (Figure 4c).
Efficient reduction of S by 1 is not anticipated unless invoking a PCET pathway, due to the
0
+
0
+
0
+
0
considerable difference in the formal potential of 1 (E ′(1 /1 ) = −1.47 V vs SCE; Figure 3b) and the
red
peak potential of the imine (E
of either 1 or 2* with TEA (pK
p
= −2.18 V vs SCE; Figure 3a). A comparison of the reductant/acid pairs
0
•+
60
a
~14.7 in acetonitrile ) reveals the green emitting complex has a
lower effective BDFE, 26.6 kcal mol , compared with 31.9 kcal mol for 1 (Table S6). Either iridium
complex results in a PCET reagent pair that may facilitate iminyl N−H bond formation (28.7 kcal mol-
-
1
-1
0
1
)
. Nevertheless, the observed evolution of green emission over time excludes the efficient reduction
+
of S by a conventional photocatalytic cycle utilizing only 1 .
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(a)
(i)
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(
ii)
(iii)
(b)
(i)
(
iii)
(ii)
(c)
0
Figure 4. (a) Emission intensity of 2 (monitored at λem = 465 nm) and (b) absorbance intensity of 1 (monitored
at λabs = 530 nm) from replicate reactions of 1 (0.2 mM) and TEA (50 mM) irradiated with blue light with the
addition of (i) 0 mM, (ii) 10 mM, or (iii) 50 mM of imine S. (c) Normalized emission spectra of 1 (orange curve),
prepared in situ (green curve), and the steady state emission spectrum (blue curve) recorded at the conclusion
+
+
2
of (iii). Exponential fits of the profiles shown in Figure 4a(i) and 4b(i) are shown in Figure S32.
Experiments were then conducted to determine if the phosphorescence of 2, prepared in situ from a
de-oxygenated solution containing 1 (0.1 mM) and TEA (50 mM), can be quenched in the presence of
S. Steady-state and time-resolved luminescence measurements resulted in linear Stern-Volmer plots
with calculated quenching rate constants of 1.2 ±0.6 × 10 M s and 1.6 ±0.3 × 10 M s , respectively
Figure 5a). These rate constants are sufficiently similar to each other to conclude that the quenching
+
6
-1 -1
6
-1 -1
(
reaction is a dynamic process involving solely the electronically excited 2* and S. Conversion of S also
exhibited a non-linear dependence with respect to irradiation intensity (Figure S33).
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0
Figure 5. (a) Stern-Volmer plot of the steady-state (red) and time-resolved (blue) emission quenching of 2,
prepared in situ, by the imine S. (b) TA spectra of 2 prepared in situ with the addition of S (50 mM) recorded at
various times after excitation by a pump laser pulse at 355 nm.
Transient absorption (TA) spectroscopy was used to further probe the nature of this dynamic
interaction. As mentioned above, the TA spectrum of 2* prepared in situ (Figure 1c) displays
absorption bands at 370 nm, 460 nm and >650 nm, along with phosphorescence centered at ~500 nm.
Addition of S to this solution resulted in a very different TA spectrum (Figure 5b). The excited state
decays rapidly, as observed by the absorption bands at 460 and >650 nm, but unlike the differential
spectrum of 2 in the absence of S (Figure S8), much of the observed absorption does not fully decay
over 10 μs, or even when the measurement time was increased to 250 μs (Figure S34). This is
consistent with the quenching of 2* via electron transfer (or PCET) with the imine substrate. Increasing
the concentration of TEA did not increase the decay rate of the persistent absorption ascribed to the
PET product of 2. Once quenched, the absorption band at 370 nm shifts to 390 nm, and a new band
at 540 nm (Figure 5b) emerges over the first 10-20 μs before remaining constant (Figure S34),
0
irrespective of S concentration. These spectral features are remarkably similar to those of 1 , as
measured by both time-resolved spectroscopy and spectroelectrochemistry (Figure 1b). These
findings, in addition to the persistence of 1 /1 observed in kinetic experiments, imply a mechanistic
23
+
0
+
+
pathway in which 2 is able to reform 1 .
On the basis of the accumulated evidence, we propose a mechanism comprising two interconnected
photoredox cycles for the light-driven reduction of imine substrates using the catalyst/electron donor
+
0
0
+
0
•+
pair 1 and TEA. The initial PET (Scheme 3, steps (i,ii)) forms 1 (E ′(1 /1 ) = −1.47 V vs SCE) and TEA .
0
Readily reducible substrates may react with 1 (step (iii)) to complete a conventional photoredox cycle,
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which would prohibit the formation of 2 (step (iv)). In the case of less reducible substrates where
electron transfer is thermodynamically unfavorable, hydrogen-atom transfer (either concerted or
stepwise) from TEA to the reduced (dtb-bpy) ligand of 1 predominates to generate 2 (step (iv)).
Previous examples of transition metal photoredox catalyst modification, such as the photo-initiated
alkylation of homoleptic iridium complexes,6 typically result in complete loss of activity.
•
+
•−
0
1-62
1
1
1
1
1
1
1
1
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1
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0
Absorption of light by 2 (step (v)) produces the excited state, 2*, which has a reduction potential
0
+
+
0
(
E ′(2 /2*) = −1.70 V vs SCE), well beyond those of 1 * and 1 . The substrate S oxidizes 2*, via electron
−
•
+
+
transfer or PCET process, to generate either S or SH and 2 (step (vi)), with 2 presumably comprising
iridium in the +4 oxidation state. When the imine is present, the small amount of 2 formed early in
the reaction creates an effective photoredox cycle (steps (v-vii)) that continuously generates 2 . This
+
0
+
0
0
+
0
oxidized intermediate (E ′(2 /2) = 0.88 V vs SCE) reacts favorably with 1 (E ′(1 /1 ) = −1.47 V vs SCE)
+
(step (vii)), to reform both 2 and 1 in a single electron transfer (step (vii)) and results in a continuous
flow of electrons between the two photoredox cycles. Although TEA is not a suitable electron donor
+
+
to reduce 2 (as observed by TA measurements), the concomitant reductive quenching of 1 * by TEA
0
•
•+
+
generates 1 and TEA (via TEA , Scheme 2b), both of which should reduce 2 (step (vii)). We cannot
+
+
rule out the possibility that 1 is regenerated following hydrogen atom transfer from 2 to acceptors
such as S or SH (step (viii)). While PCET from metal complexes is a known phenomenon it seems
unlikely to occur from the semi-saturated heterocycle present in 2.
•
−
•
63
+
+
Scheme 3. Proposed interconnected photoredox mechanism: (i) Photoexcitation of [Ir(ppy) (dtb-bpy)] , 1 ; (ii)
2
reductive quenching of the electronically excited catalyst by TEA; (iii) completion of a conventional photoredox
cycle with a readily reduced substrate; (iv) generation of new catalyst, 2, via (concerted or stepwise) hydrogen
transfers from TEA• onto the dtb-bpy•− ligand of 1 ; (v) photoexcitation of the new catalyst to form a more
+
0
powerful reductant; (vi) reduction of substrate through oxidative quenching of the electronically excited
0
•
catalyst; (vii) completion of the new catalytic cycle upon reaction with 1 (or TEA ); (viii) completion of the dual
catalytic cycle via hydrogen abstraction from 2 by the reduced substrate; and finally (ix) product, P, formation.
[
H] denotes formal addition of a hydrogen atom that may occur via one of three pathways; hydrogen atom
abstraction, concerted proton and electron transfer, step-wise proton and electron transfer. Reduction
potentials for all species depicted in this scheme are compiled in Table S5.
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6
Applicability to other energy demanding substrates
+
To gain further insight into the generality of the enhanced reactivity of 1 (via formation of 2), we
examined the formation of aryl radicals from aryl halide precursors. Under photoredox conditions,
these reactions are usually restricted to electron-poor substrates due to the limited reducing power
of conventional catalysts. The homoleptic Ir(ppy) catalyst, for example, is known to reduce aryl
Hydrodehalogenation and cyclization of alkyl iodides
using catalyst 1 via a reductive quenching cycle using DIPEA as an electron donor has also been
64
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0
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65-66
iodides via an oxidative quenching cycle.
+
2
2, 67
reported.
After a preliminary exploration of the mechanism, and noting the large difference in
red
≤ −2.5 V vs SCE) and catalyst 1+
reduction potentials between various unactivated alkyl iodides (E
p
0
+
0
(
E ′(1 /1 ) = −1.47 V vs SCE), the authors postulated a radical chain propagation process and exciplex
formation between substrate and sacrificial amine (possibly accelerated by energy transfer from the
catalyst), but they could not rule out the participation of alternative light-consuming events.
Applying our photocatalytic conditions to a range of aryl iodides, bromides and chlorides afforded the
corresponding dehalogenated products in excellent yields (Table 1). The disubstituted 1-iodo-4-
bromobenzene was converted to the mono-substituted bromobenzene in a 93% yield, with a further
4% of the doubly reduced benzene product detected. Importantly, no product was detected in the
absence of either photocatalyst 1 , light or TEA (Table S7). Although the irreversible reductive peak
potentials of the substrates (Table S8) cannot be accurately compared to the reversible or quasi-
+
6
8-69
reversible redox potentials of the metal complexes,
we note that the excited state potential for
*, estimated as E ′(2 /2*) = −1.7 V vs SCE, is similar to that of the well-known highly reductive
0
+
2
0
+
1
photocatalyst Ir(ppy)
3
* (E ′(Ir /Ir*) = −1.73 V vs SCE (Scheme 1a), so that the reductive scope of the
+
tandem photoredox cycle (involving 1 and 2; Scheme 3) extends at least to a potential similar to that
red
of Ir(ppy)
3
. We were surprised to observe excellent reactivity with highly deactivated substrates, E
p
>
−2.2 V vs SCE, and subsequent emission quenching experiments revealed an interaction between
= −2.72 V vs SCE; Figure S35). This may in part be
rationalized by the fundamental error in referencing the cathodic peak potentials of organic substrate
red
the excited state 2* and 4-bromoanisole (E
p
70
reductions compared to standard reduction potentials, and the estimation of excited state
71
potentials, but interactions that reduce the energy requirements of the PET cannot be ruled out.
CONCLUSIONS
6
Transition metal photoredox catalysts (particularly d iridium(III) and ruthenium(II) complexes) have
long been considered purely as electron shuttles between organic substrates. The present work
reveals that this is not always the case. Irradiation of the iridium complex 1 , a common photoredox
catalyst, in the presence of TEA leads to its chemical transformation into a new iridium-based
analogue, 2, via the PET product 1 . The structure of 2 is distinguished by the semi-saturation of the
dtb-bpy ancillary ligand, with hydrogens sourced from the trialkylamine. This new complex is capable
of reducing more deactivated substrates than 1 , and helps explain the previously observed reactivity
of substrates that possess reduction potentials inaccessible by any redox state of 1 . Luminescence
+
0
+
+
quenching and TA spectroscopy experiments revealed that a representative imine substrate interacts
solely with the excited state 2* via an electron transfer process, with subsequent steps leading to the
+
regeneration of 1 .
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The combined evidence supports the existence of two distinct but inextricably linked photoredox
cycles, allowing the engagement of substrates with a wide range of energy requirements. This is the
first known example of dual oxidative and reductive photoredox cycles operating in tandem, and the
utility of this pathway was further demonstrated through the efficient reduction of aryl and alkyl
halides that would otherwise have been considered thermodynamically unfeasible based solely on a
+
single cycle involving 1 . This work extends the understanding of the mechanism of iridium-based
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
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photoredox processes and its application in the design and development of new catalysts and
synthetic methodology is currently the subject of further investigation.
+
Table 1. Application of 1 and TEA for the reductive dehalogenation of selected aryl and alkyl halides.
EXPERIMENTAL SECTION
General. Unless otherwise stated, reagents and solvents were purchased from commercial
sources and used without further purification. [Ir(ppy)
2
(dtb-bpy)]PF and N-(diphenylmethylene)-1-
6
23
phenylmethanamine (S) were prepared as previously reported. The organic soluble tetra-N-
butylammonium oxalate salt was prepared as previously reported.7 Acetonitrile was distilled over
calcium hydride under a nitrogen atmosphere and collected as needed. Triethylamine (TEA) was
distilled over potassium hydroxide pellets under a nitrogen atmosphere and stored under nitrogen in
an amber bottle.
2-74
1H NMR spectra were recorded using a Bruker 400 Ultrashield (400 MHz) spectrometer. Chemical
shifts are expressed in parts per million (ppm) and referenced to internal solvent peaks. Electrospray
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ionisation-mass spectrometry (ESI-MS) experiments were performed using a Thermo Scientific Q
Exactive mass spectrometer fitted with a HESI-II ion source. Positive ion electrospray mass spectra
were recorded in an appropriate mass range set for 140,000 mass resolution. Samples were
introduced by direct infusion using a syringe pump set at 5 μL min as acetonitrile solutions. The
nitrogen nebulizing/desolvation gas used was heated to 250°C, the sheath gas flow rate was set to 5
and the auxiliary gas flow rate to 0 (both arbitrary units). The spray voltage was 4.50 kV and the
capillary temperature was 260°C. MS/MS experiments were performed by selecting the precursor ion
and subjecting it to a normalized collision energy of 35 eV in a higher-energy collisional dissociation
(HCD) cell.
UV-Vis absorbance spectroscopy. Absorption spectra were measured using a Perkin Elmer
Lambda 1050 spectrometer or a Cary 300 Bio UV/Vis spectrophotometer. Transient absorption
spectra were recorded using an Edinburgh Instruments LP900 spectrometer coupled with a
nanosecond pulsed Nd -YAG laser (Quantel, Brio), operating at 355 nm and 10 Hz repetition rate as
the excitation source.
Photoluminescence spectroscopy. Steady-state spectra were measured using a Cary Eclipse
spectrofluorometer (5 nm band pass, 1 nm data interval, PMT voltage: 600 V). Spectra were corrected
for the change in instrument sensitivity across the wavelength range, using a correction factor
established using a quartz halogen tungsten lamp of standard spectral irradiance. Low temperature
spectra were obtained using a Cary Eclipse spectrofluorometer and an OptistatDN Variable
Temperature Liquid Nitrogen Cryostat (Oxford Instruments) with custom-made quartz sample
-
1
0
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2
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0
3
+
75
holder. Spectra were corrected as described above. Spectra were collected at 85 K to avoid damage
7
6-77
to the spectroscopic cuvettes near 77 K.
for complexes such as [Ru(bpy)
No difference was observed in the max at 77 K and 85 K
under our instrumental conditions. Additional steady-
state spectra and time-resolved emission spectra were measured using an Edinburgh Instruments
FLSP920 spectrometer equipped with a 378 nm pulsed diode laser excitation source. The value
obtained for the lifetime of 1 in de-oxygenated acetonitrile in the present work of 0.57 μs is consistent
with other reported values: 0.62 μs, 0.56 μs, and 0.55 μs.
Electrochemistry and electrochemiluminescence (ECL). The cell comprised a cylindrical glass
vessel with a flat base and custom-built Teflon lid with holes to fit the electrodes (glassy carbon
working (CH Instruments), platinum wire counter and either silver wire or ‘leakless’ Ag/AgCl (eDAQ)
reference). The cell was housed in a light-tight Faraday cage. Potentials were applied using an Autolab
2+
75
3
] and Ir(ppy)
3
+
78
79
80
81
+/0
PGSTAT204 potentiostat, and referenced in situ to the ferrocenium/ferrocene (Fc ) couple, and
converted to the reported reference (SCE) using Fc+ = +0.4 V. Freshly distilled acetonitrile with
/0
82
0
.1 M TBAPF supporting electrolyte was used as the solvent. Electrodes were cleaned/polished prior
6
to each experiment. The working electrodes were polished using 0.05 m alumina powder on a felt
pad with deionized water. All electrodes were rinsed with deionized water, ethanol and acetone. The
solvent was degassed with nitrogen or argon for 5 minutes. Additional electrochemical experiments
were conducted using the flow system described in the Supporting Information.
The intensity of light emanating from electronically excited species formed in reactions between
intermediates generated at the working electrode surface were measured by positioning a
photomultiplier tube (PMT) below the flat transparent base of the electrochemical cell described
above. ECL spectra were measured by replacing the PMT with a collimating lens (Ocean Optics 74-UV,
00–2000 nm) vertically aligned with the face of the working electrode and fitted to an optical fiber
1.0 m length, 1.0 mm core diameter) to transfer light to an Ocean Optics QEpro CCD spectrometer.
Acquisition was triggered using a HR 4000 Break-Out box in conjunction with the potentiostat. Under
the configuration employed, the effective spectral resolution was 30 nm (FWHM). The color of the
ECL was recorded by instead positioning a Canon EOS 6D DSLR camera fitted with a Tonika AT-X PRO
MACRO 100 mm f/2.8 D lens below the base of the electrochemical cell, within the purpose-built light-
2
(
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tight Faraday cage. The camera shutter activation was controlled by the potentiostat via the
configurable DIO port, using a simple transistor switch and relay to electronically control the remote
shutter release.
Kinetics Measurements. Using a peristaltic pump (Gilson Minipuls 3), reaction solutions were
aspirated from a photoredox chamber via two flow-through quartz cuvettes and returned the
chamber (Figure S2). The cuvettes had three windows to enable either absorbance or
photoluminescence measurements, obtained using the spectrometers above. The distance of tubing
1
1
1
1
1
1
1
1
1
1
2
2
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2
2
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-
1
(
0.8 mm i.d. PTFE) from the reaction chamber to the cuvette was 0.4 m and flow rate was 3.5 mL min ,
such that solution was moved to the cuvette in 6 s. For the electrochemical reduction, a bulk
electrolysis cell kit (BASi) was used. The chamber was fitted with three electrodes: a reticulated
vitreous carbon working, a Pt wire counter in a fritted cell containing 0.1 M TBAPF in acetonitrile, and
a ‘leakless’ Ag/AgCl reference. To reduce 1 to 1 , a potential of -1.8 V vs SCE was applied at the
working electrode.
Imine conversion as a function of light intensity was measured using a Kessil PR160 lamp (467
nm, 40 W) operated at different power settings. Reagents were combined in acetonitrile in a septum
capped sample vial, degassed for three minutes and then irradiated for 15 minutes. Substrate
6
+
0
1
conversion was determined by H NMR spectroscopy. Reported values are the average of duplicate
runs.
Photoredox catalysis screening. Reactions were performed using a custom-built blue LED
photoreactor. The reactor was comprised of a rectangular metal frame (100 mm × 110 mm) with 14
blue LEDs (2 × SP-02-V4 LED assembly, each consisting of 7 LXML-PR02-A900 RoyalBlue LUXEON Rebel
ES LEDs; the spectral output of the LEDs spans the 400–500 nm range with a maximum intensity at
447 nm. www.luxeonstar.com). The photoreactor was fitted with PC cooling fans and heat sinks for
temperature control (internal temperature of the reactor typically reaches 27±5°C overnight). The
power of the light source can be varied using an intensity control. The emission output of the light
source used was measured using a Princeton Instruments PIXIS 1024BR CCD Detector. The
measurement was conducted by shining the LED light indirectly into the aperture of the detector. The
dependence of product formation as a function of light intensity was performed using a standard PR-
1
60-456 Kessil® lamp with four levels of intensity control (www.kessil.com). Additional photoredox
catalysis experiments were conducted using the flow system described in the Supporting Information.
General method for reductive dehalogenation of organohalides. The appropriate aryl or alkyl
halide (0.1 mmol) and [Ir(ppy)
dissolved in deuterated acetonitrile (1 mL). Triethylamine (1.0 mmol) was added, and the reaction
mixture sparged with N for 10 min at 0°C to avoid evaporation of volatile materials. The cap was
2
(dtb-bpy)]PF (1.5 mol %) were placed in a 4 mL pressure relief vial and
6
2
wrapped with Teflon tape and parafilm, and the reaction mixture stirred at room temperature for 16
hours, whilst being irradiated with 40 W blue LED light. Sulfolene (0.1 mmol) was added as an internal
standard and the reaction mixture analysed directly by H NMR spectroscopy (see supporting
information for representative spectra). Control experiments were prepared according to the same
procedure without the inclusion of the appropriate reagent (photocatalyst, amine or light). These
reaction mixtures were analysed directly by H NMR spectroscopy using 2,5-dimethylfuran (0.1 mmol)
1
1
as an internal standard (see Figures S36-S37).
Computational approach. DFT calculations were carried out within the Gaussian 16 suite of
programs. Ground state singlet and triplet geometries as well as redox states were optimized in the
presence of solvent with the mPW1PW91 functional in conjunction with the def2-SVP basis set and
associated core potential. Frequency calculations were performed at the same level of theory and
confirmed that all structures are minima with no imaginary frequencies. The polarizable continuum
model (PCM) self-consistent reaction field (SCRF) was used to model solvent effects with Truhlar’s
SMD solvent model, with a solvent of acetonitrile for consistency with the experimental system.
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Molecular orbitals were calculated with BP8688-89 and a def2-TZVP basis set (Tables S9-S11 and Figure
Molecular orbital analysis was carried out
with the QMForge program. TD-DFT calculations were performed with the long-range corrected
7
6, 90
S38), which has been shown to produce reliable results.
91
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method B97XD in conjunction with the def2-SVP basis set.
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ACKNOWLEDGEMENTS
This work was funded by the Australian Research Council (DP160103046, FT140100514,
IC170100020). AP acknowledges funding from the University of Melbourne and CSIRO joint
Establishment Grant. We acknowledge generous allocations of computing from La Trobe University,
Intersect, and NCI. The authors thank Ms Lifen Chen (Deakin University) for assistance with obtaining
low-temperature photoluminescence spectra. The authors would also like to thank Dr Jo Cosgriff and
Dr Carl Braybook (CSIRO) for assistance with the NMR and ESI-MS experiments.
SUPPORTING INFORMATION
Description of equipment, extra experimental data and numerical calculations
REFERENCES
1
2
3
.
.
.
Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C., Visible light photoredox catalysis with transition
metal complexes: applications in organic synthesis. Chem. Rev. 2013, 113, 5322-5363.
Larsen, C. B.; Wenger, O. S., Photoredox Catalysis with Metal Complexes Made from Earth-
Abundant Elements. Chem. Eur. J. 2018, 24, 2039-2058.
Chen, M.; Zhong, M.; Johnson, J. A., Light-Controlled Radical Polymerization: Mechanisms,
Methods, and Applications. Chem. Rev. 2016, 116, 10167-211.
4.
Corrigan, N.; Shanmugam, S.; Xu, J.; Boyer, C., Photocatalysis in organic and polymer synthesis.
Chem. Soc. Rev. 2016, 45, 6165-6212.
5
6
7
.
.
.
Karkas, M. D.; Porco, J. A., Jr.; Stephenson, C. R., Photochemical Approaches to Complex
Chemotypes: Applications in Natural Product Synthesis. Chem. Rev. 2016, 116, 9683-747.
Marzo, L.; Pagire, S. K.; Reiser, O.; Konig, B., Visible-Light Photocatalysis: Does It Make a
Difference in Organic Synthesis? Angew. Chem. Int. Ed. 2018, 57, 10034-10072.
Phommalysack-Lovan, J.; Chu, Y.; Boyer, C.; Xu, J., PET-RAFT polymerisation: towards green and
precision polymer manufacturing. Chem. Commun. 2018, 54, 6591-6606.
Shaw, M. H.; Twilton, J.; MacMillan, D. W., Photoredox Catalysis in Organic Chemistry. J. Org.
Chem. 2016, 81, 6898-926.
8.
9
1
1
.
Wang, C. S.; Dixneuf, P. H.; Soule, J. F., Photoredox Catalysis for Building C-C Bonds from
C(sp(2))-H Bonds. Chem. Rev. 2018, 118, 7532-7585.
0. Hedstrand, D. M.; Kruizinga, W. H.; Kellogg, R. M., Light induced and dye accelerated reductions
of phenacyl onium salts by 1,4-dihydropyridines. Tetrahedron Lett. 1978, 1255-1258.
1. Arias-Rotondo, D. M.; McCusker, J. K., The photophysics of photoredox catalysis: a roadmap for
catalyst design. Chem. Soc. Rev. 2016, 45, 5803-5820.
12. Buzzetti, L.; Crisenza, G. E. M.; Melchiorre, P., Mechanistic Studies in Photocatalysis. Angew.
Chem. Int. Ed. 2018, 58, 3730-3747.
13. Kerzig, C.; Goez, M., Combining energy and electron transfer in a supramolecular environment
for the "green" generation and utilization of hydrated electrons through photoredox catalysis.
Chem. Sci. 2016, 7, 3862-3868.
ACS Paragon Plus Environment

Page 19 of 24
Journal of the American Chemical Society
1
9
1
2
3
4
5
6
7
8
9
1
1
1
4. Ghosh, I.; Shaikh, R. S.; König, B., Sensitization-initiated electron transfer for photoredox
catalysis. Angew. Chem. Int. Ed. 2017, 56, 8544-8549.
5. Marchini, M.; Bergamini, G.; Cozzi, P. G.; Ceroni, P.; Balzani, V., Photoredox catalysis: the need
to elucidate the photochemical mechanism. Angew. Chem. Int. Ed. 2017, 56, 2-4.
6. Ghosh, I.; Shaikh, R. S.; König, B., Reply to "Photoredox catalysis: the need to elucidate the
photochemical mechanism". Angew. Chem. Int. Ed. 2017, 56, 12822-12824.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
17. Gentry, E. C.; Knowles, R. R., Synthetic Applications of Proton-Coupled Electron Transfer. Acc.
Chem. Res. 2016, 49, 1546-1556.
1
1
2
8. Hoffmann, N., Proton-Coupled Electron Transfer in Photoredox Catalytic Reactions. Eur. J. Org.
Chem. 2017, 2017, 1982-1992.
9. Warren, J. J.; Tronic, T. A.; Mayer, J. M., Thermochemistry of proton-coupled electron transfer
reagents and its implications. Chem. Rev. 2010, 110, 6961-7001.
0. Nicewicz, D.; Roth, H.; Romero, N., Experimental and Calculated Electrochemical Potentials of
Common Organic Molecules for Applications to Single-Electron Redox Chemistry. Synlett 2015,
27, 714-723.
21. Nakajima, M.; Fava, E.; Loescher, S.; Jiang, Z.; Rueping, M., Photoredox-Catalyzed Reductive
Coupling of Aldehydes, Ketones, and Imines with Visible Light. Angew. Chem. Int. Ed. 2015, 54,
8
828-32.
2
2
2. Shen, Y.; Cornella, J.; Julia-Hernandez, F.; Martin, R., Visible-Light-Promoted Atom Transfer
Radical Cyclization of Unactivated Alkyl Iodides. ACS Catal. 2017, 7, 409-412.
3. van As, D. J.; Connell, T. U.; Brzozowski, M.; Scully, A. D.; Polyzos, A., Photocatalytic and
Chemoselective Transfer Hydrogenation of Diarylimines in Batch and Continuous Flow. Org.
Lett. 2018, 20, 905-908.
24. Curtin, P. N.; Tinker, L. L.; Burgess, C. M.; Cline, E. D.; Bernhard, S., Structure-activity correlations
among iridium(III) photosensitizers in a robust water-reducing system. Inorg. Chem. 2009, 48,
1
0498-10506.
5. Wang, J.; Li, C.; Zhou, Q.; Wang, W.; Hou, Y.; Zhang, B.; Wang, X., A polypyridyl Co(II) complex-
based water reduction catalyst with double H evolution sites. Catal. Sci. Technol. 2016, 6, 8482-
489.
2
2
2
8
6. Tucker, J. W.; Nguyen, J. D.; Narayanam, J. M. R.; Krabbe, S. W.; Stephenson, C. R. J., Tin-free
radical cyclization reactions initiated by visible light photoredox catalysis. Chem. Commun.
2010, 46, 4985-4987.
27. Courant, T.; Masson, G., Photoredox-Initiated α-Alkylation of Imines through a Three-
Component Radical/Cationic Reaction. Chem. Eur. J. 2012, 18, 423-427, S423/1-S423/36.
2
2
3
8. Duan, Z.; Li, W.; Lei, A., Nickel-Catalyzed Reductive Cross-Coupling of Aryl Bromides with Alkyl
Bromides: Et3N as the Terminal Reductant. Org. Lett. 2016, 18, 4012-4015.
9. Ren, X.; Guo, Q.; Chen, J.; Xie, H.; Xu, Q.; Lu, Z., Visible-Light Promoted Diastereodivergent
Intramolecular Oxyamidation of Alkenes. Chem. Eur. J. 2016, 22, 18695-18699.
0. Alpers, D.; Brasholz, M.; Rehbein, J., Photoredox-Induced Radical 6-exo-trig Cyclizations onto
the Indole Nucleus: Aromative versus Dearomative Pathways. Eur. J. Org. Chem. 2017, 2017,
2186-2193.
3
3
1. Chen, M.; Zhao, X.; Yang, C.; Xia, W., Visible-Light-Triggered Directly Reductive Arylation of
Carbonyl/Iminyl Derivatives through Photocatalytic PCET. Org. Lett. 2017, 19, 3807-3810.
2. Wang, R.; Ma, M.; Gong, X.; Panetti, G. B.; Fan, X.; Walsh, P. J., Visible-light-mediated umpolung
reactivity of imines: ketimine reductions with Cy
436.
2
NMe and water. Org. Lett. 2018, 20, 2433-
2
3
3. Wu, S.-H.; Ling, J.-W.; Lai, S.-H.; Huang, M.-J.; Cheng, C. H.; Chen, I. C., Dynamics of the Excited
+
States of [Ir(ppy)
2
bpy] with Triple Phosphorescence. J. Phys. Chem. A 2010, 114, 10339-10344.
34. Alrawashdeh, L. R.; Cronin, M. P.; Woodward, C. E.; Day, A. I.; Wallace, L., Iridium cyclometalated
complexes in host-guest chemistry: a strategy for maximizing quantum yield in aqueous media.
Inorg. Chem. 2016, 55, 6759-6769.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 24
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3
3
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4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
bpy]+
3
3
5. Yen, H.-Y.; Huang, M.-J.; Chen, I. C., Physical properties of triplet excited states of [Ir(ppy)
2
in polar solvent and in nonaqueous confined reversed micelle. J. Photochem. Photobiol. A 2016,
18, 33-41.
6. Ghosh, I.; Ghosh, T.; Bardagi, J. I.; Koenig, B., Reduction of aryl halides by consecutive visible
3
light-induced electron transfer processes. Science 2014, 346, 725-728.
37. Ghosh, I.; Koenig, B., Chromoselective Photocatalysis: Controlled Bond Activation through Light-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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3
4
5
6
7
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9
0
1
2
3
4
5
6
7
8
9
0
Color Regulation of Redox Potentials. Angew. Chem. Int. Ed. 2016, 55, 7676-7679.
3
3
4
8. Graml, A.; Ghosh, I.; Koenig, B., Synthesis of Arylated Nucleobases by Visible Light Photoredox
Catalysis. J. Org. Chem. 2017, 82, 3552-3560.
9. Shaikh, R. S.; Duesel, S. J. S.; Koenig, B., Visible-Light Photo-Arbuzov Reaction of Aryl Bromides
and Trialkyl Phosphites Yielding Aryl Phosphonates. ACS Catal. 2016, 6, 8410-8414.
0. Zeng, L.; Liu, T.; He, C.; Shi, D.; Zhang, F.; Duan, C., Organized Aggregation Makes Insoluble
Perylene Diimide Efficient for the Reduction of Aryl Halides via Consecutive Visible Light-
Induced Electron-Transfer Processes. J. Am. Chem. Soc. 2016, 138, 3958-61.
41. Haimerl, J. M.; Ghosh, I.; Kꢀnig, B.; Lupton, J. M.; Vogelsang, J., Chemical Photocatalysis with
Rhodamine 6G: Investigation of Photoreduction by Simultaneous Fluorescence Correlation
Spectroscopy and Fluorescence Lifetime Measurements. J. Phys. Chem. B 2018, 122, 10728-
1
0735.
4
4
2. Haimerl, J.; Ghosh, I.; König, B.; Vogelsang, J.; Lupton, J. M., Single-molecule photoredox
catalysis. Chem. Sci. 2019, 10, DOI: 10.1039/C8SC03860K.
3. Marchini, M.; Gualandi, A.; Mengozzi, L.; Franchi, P.; Lucarini, M.; Cozzi, P. G.; Balzani, V.; Ceroni,
P., Mechanistic insights into two-photon-driven photocatalysis in organic synthesis. Phys. Chem.
Chem. Phys. 2018, 20, 8071-8076.
44. Ichimura, K.; Kobayashi, T.; King, K. A.; Watts, R. J., Excited-state absorption spectroscopy of
ortho-metalated iridium(III) complexes. J. Phys. Chem. 1987, 91, 6104-6106.
4
4
5. Miller, K. L.; Williams, B. N.; Benitez, D.; Carver, C. T.; Ogilby, K. R.; Tkatchouk, E.; Goddard, W.
A.; Diaconescu, P. L., Dearomatization Reactions of N-Heterocycles Mediated by Group 3
Complexes. J. Am. Chem. Soc. 2010, 132, 342-355.
6. Weetman, C.; Hill, M. S.; Mahon, M. F., Magnesium-catalysed hydroboration of pyridines:
Kinetic analysis and poly-pyridine dearomatisation. Polyhedron 2016, 103, 115-120.
47. Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, L. J.; Milstein, D., Efficient hydrogenation of
organic carbonates, carbamates and formates indicates alternative routes to methanol based
on CO and CO. Nat. Chem. 2011, 3, 609-14.
2
4
4
5
8. Zell, T.; Milstein, D., Hydrogenation and dehydrogenation iron pincer catalysts capable of metal-
ligand cooperation by aromatization/dearomatization. Acc. Chem. Res. 2015, 48, 1979-94.
9. Arevalo, R.; Riera, L.; Perez, J., Interligand C-C Coupling between alpha-Methyl N-Heterocycles
and bipy or phen at Rhenium Tricarbonyl Complexes. Inorg. Chem. 2017, 56, 4249-4252.
0. Hu, J.; Wang, J.; Nguyen, T. H.; Zheng, N., The chemistry of amine radical cations produced by
visible light photoredox catalysis. Beilstein J. Org. Chem. 2013, 9, 1977-2001.
51. Sato, S.; Morikawa, T.; Kajino, T.; Ishitani, O., A highly efficient mononuclear iridium complex
photocatalyst for CO2 reduction under visible light. Angew. Chem. Int. Ed. 2013, 52, 988-992.
2. Mann, C. K.; Barnes, K. K., Electrochemical Reactions in Non-Aqueous Systems. Marcel Dekker:
New York, 1970; p 560.
3. Kim, S.; Park, G.; Cho, E. J.; You, Y., Coreactant strategy for the photoredox catalytic generation
of trifluoromethyl radicals under low-energy photoirradiation. J. Org. Chem. 2016, 81, 7072-
5
5
7
079.
5
4. Barbante, G. J.; Kebede, N.; Hindson, C. M.; Doeven, E. H.; Zammit, E. M.; Hanson, G. R.; Hogan,
C. F.; Francis, P. S., Control of excitation and quenching in multi-colour electrogenerated
chemiluminescence systems through choice of co-reactant. Chem. Eur. J. 2014, 20, 14026-
1
4031.
ACS Paragon Plus Environment

Page 21 of 24
Journal of the American Chemical Society
2
1
1
2
3
4
5
6
7
8
9
5
5
5
5. Liu, Z.; Qi, W.; Xu, G., Recent advances in electrochemiluminescence. Chem. Soc. Rev. 2015, 44,
117-3142.
6. Hu, L.; Xu, G., Applications and trends in electrochemiluminescence. Chem. Soc. Rev. 2010, 39,
275-3304.
7. Bhattacharyya, K.; Das, P. K., Nanosecond transient processes in the triethylamine quenching of
benzophenone triplets in aqueous alkaline media. Substituent effect, ketyl radical
deprotonation, and secondary photoreduction kinetics. J. Phys. Chem. 1986, 90, 3987-3993.
3
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
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3
3
3
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4
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4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
5
6
6
8. Miao, W.; Choi, J.-P.; Bard, A. J., Electrogenerated chemiluminescence 69: the tris(2,2'-
2+
bipyridine)ruthenium(II), (Ru(bpy)
3
)/tri-n-propylamine (TPrA) system revisited - a new route
involving TPrA cation radicals. J. Am. Chem. Soc. 2002, 124, 14478-14485.
9. Hindson, C. M.; Hanson, G. R.; Francis, P. S.; Adcock, J. L.; Barnett, N. W., Any old radical won't
2
+
do: an EPR study of the selective excitation and quenching mechanisms of [Ru(bipy) ]
3
chemiluminescence and electrochemiluminescence. Chem. Eur. J. 2011, 17, 8018-8022.
0. Beatty, J. W.; Stephenson, C. R., Amine Functionalization via Oxidative Photoredox Catalysis:
Methodology Development and Complex Molecule Synthesis. Acc. Chem. Res. 2015, 48, 1474-
84.
1. Devery, J. J.; Douglas, J. J.; Nguyen, J. D.; Cole, K. P.; Flowers, R. A.; Stephenson, C. R. J., Ligand
functionalization as a deactivation pathway in a fac-Ir(ppy)3-mediated radical addition. Chem.
Sci. 2015, 6, 537-541.
2. Stephenson, C.; Swift, E.; Williams, T., Intermolecular Photocatalytic C–H Functionalization of
Electron-Rich Heterocycles with Tertiary Alkyl Halides. Synlett 2016, 27, 754-758.
3. Lennox, J. C.; Kurtz, D. A.; Huang, T.; Dempsey, J. L., Excited-State Proton-Coupled Electron
Transfer: Different Avenues for Promoting Proton/Electron Movement with Solar Photons. ACS
Energy Lett. 2017, 2, 1246-1256.
4. Shon, J.-H.; Sittel, S.; Teets, T. S., Synthesis and Characterization of Strong Cyclometalated
Iridium Photoreductants for Application in Photocatalytic Aryl Bromide Hydrodebromination.
ACS Catal. 2019, 8646-8658.
6
6
6
6
5. Nguyen, J. D.; D'Amato, E. M.; Narayanam, J. M. R.; Stephenson, C. R. J., Engaging unactivated
alkyl, alkenyl and aryl iodides in visible-light-mediated free radical reactions. Nat. Chem. 2012,
4
, 854.
66. Czyz, M. L.; Weragoda, G. K.; Monaghan, R.; Connell, T. U.; Brzozowski, M.; Scully, A. D.; Burton,
J.; Lupton, D. W.; Polyzos, A., A visible-light photocatalytic thiolation of aryl, heteroaryl and vinyl
iodides. Org. Biomol. Chem. 2018, 16, 1543-1551.
6
6
6
7. Kim, H.; Lee, C., Visible-light-induced photocatalytic reductive transformations of
organohalides. Angew. Chem. Int. Ed. 2012, 51, 12303-12306.
8. Nelsen, S. F.; Hintz, P. J., Electrochemical oxidation of tertiary amines. The effect of structure
upon reversibility. J. Am. Chem. Soc. 1972, 94, 7114-7117.
9. Lai, R. Y.; Bard, A. J., Electrogenerated chemiluminescence. 70. The application of ECL to
determine electrode potentials of tri-n-propylamine, its radical cation, and intermediate free
radical in MeCN/benzene solutions. J. Phys. Chem. A 2003, 107, 3335-3340.
70. Roth, H. G.; Romero, N. A.; Nicewicz, D. A., Experimental and calculated electrochemical
potentials of common organic molecules for applications to single-electron redox chemistry.
Synlett 2016, 27, 714-723.
7
1. Jones, W. E., Jr.; Fox, M. A., Determination of excited-state redox potentials by phase-
modulated voltammetry. J. Phys. Chem. 1994, 98, 5095-5099.
2. Chang, M.-M.; Saji, T.; Bard, A. J., Electrogenerated chemiluminescence. 30. Electrochemical
oxidation of oxalate ion in the presence of luminescers in acetonitrile solutions. J. Am. Chem.
Soc. 1977, 99, 5399-5403.
7
73. Qi, H.; Chang, J.; Abdelwahed, S. H.; Thakur, K.; Rathore, R.; Bard, A. J., Electrochemistry and
electrogenerated chemiluminescence of π-Stacked poly(fluorenemethylene) oligomers.
Multiple, interacting electron transfers. J. Am. Chem. Soc. 2012, 134, 16265-16274.
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Page 22 of 24
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6
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5
5
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5
5
5
5
5
6
7
7
4. Barclay, L. R. C.; Dakin, K. A.; Zahalka, H. A., Ascorbate: antioxidant activity determined in a
hydrocarbon phase using quaternary ammonium salts as phase-transfer agents. Can. J. Chem.
1
992, 70, 2148-53.
5. Soulsby, L. C.; Hayne, D. J.; Doeven, E. H.; Wilson, D. J. D.; Agugiaro, J.; Connell, T. U.; Chen, L.;
Hogan, C. F.; Kerr, E.; Adcock, J. L.; Donnelly, P. S.; White, J. M.; Francis, P. S., Mixed annihilation
electrogenerated chemiluminescence of iridium(III) complexes. Phys. Chem. Chem. Phys. 2018,
20, 18995-19006.
6. Chen, L.; Doeven, E. H.; Wilson, D. J. D.; Kerr, E.; Hayne, D. J.; Hogan, C. F.; Yang, W.; Pham, T.
T.; Francis, P. S., Co-reactant electrogenerated chemiluminescence of iridium(III) complexes
containing an acetylacetonate ligand. ChemElectroChem 2017, 4, 1797-1808.
7. Mallet, C.; Bolduc, A.; Bishop, S.; Gautier, Y.; Skene, W. G., Unusually high fluorescence quantum
yield of a homopolyfluorenylazomethine – towards a universal fluorophore. Phys. Chem. Chem.
Phys. 2014, 16, 24382-24390.
8. Lowry, M. S.; Hudson, W. R.; Pascal, R. A., Jr.; Bernhard, S., Accelerated luminophor discovery
through combinatorial synthesis. J. Am. Chem. Soc. 2004, 126, 14129-14135.
79. Slinker Jason, D.; Gorodetsky Alon, A.; Lowry Michael, S.; Wang, J.; Parker, S.; Rohl, R.; Bernhard,
S.; Malliaras George, G., Efficient yellow electroluminescence from a single layer of a
cyclometalated iridium complex. J. Am. Chem. Soc. 2004, 126, 2763-2767.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
7
7
7
8
0. Tordera, D.; Delgado, M.; Orti, E.; Bolink, H. J.; Frey, J.; Nazeeruddin, M. K.; Baranoff, E., Stable
green electroluminescence from an iridium tris-heteroleptic ionic complex. Chem. Mater. 2012,
2
4, 1896-1903.
8
1. Kerr, E.; Doeven, E. H.; Barbante, G. J.; Hogan, C. F.; Hayne, D. J.; Donnelly, P. S.; Francis, P. S.,
New perspectives on the annihilation electrogenerated chemiluminescence of mixed metal
complexes in solution Chem. Sci. 2016, 7, 5271-5279.
8
8
2. Connelly, N. G.; Geiger, W. E., Chemical redox agents for organometallic chemistry. Chem. Rev.
1
996, 96, 877-910.
3. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.;
Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A.
F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.;
Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang,
W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda,
Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi,
R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi,
M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma,
K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, Revision A.03, Gaussian, Inc.: Wallingford
CT, 2016.
4. Adamo, C.; Barone, V., Exchange functionals with improved long-range behavior and adiabatic
connection methods without adjustable parameters: the mPW and mPW1PW models. J. Chem.
Phys. 1998, 108, 664-675.
5. Weigend, F.; Ahlrichs, R., Balanced basis sets of split valence, triple zeta valence and quadruple
zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys.
8
8
2
005, 7, 3297-3305.
8
8
6. Tomasi, J.; Mennucci, B.; Cammi, R., Quantum mechanical continuum solvation models. Chem.
Rev. 2005, 105, 2999-3093.
7. Marenich, A. V.; Cramer, C. J.; Truhlar, D. G., Universal solvation model based on solute electron
density and a continuum model of the solvent defined by the bulk dielectric constant and atomic
surface tensions. J. Phys. Chem. B 2009, 113, 6378-6396.
8
8. Becke, A. D., Density-functional exchange-energy approximation with correct asymptotic
behavior. Phys. Rev. A 1988, 38, 3098-3100.
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9. Perdew, J. P., Density-functional approximation for the correlation energy of the
inhomogeneous electron gas. Phys. Rev. B 1986, 33, 8822-8824.
0. Barbante, G. J.; Doeven, E. H.; Kerr, E.; Connell, T. U.; Donnelly, P. S.; White, J. M.; Lópes, T.;
Laird, S.; Hogan, C. F.; Wilson, D. J. D.; Barnard, P. J.; Francis, P. S., Understanding
electrogenerated chemiluminescence efficiency in blue-shifted iridium(III)-complexes: an
experimental and theoretical study. Chem. Eur. J. 2014, 20, 3322-3332.
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4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
91. Tenderholt, A. L. QMForge, Version 2.4, http://qmforge.sourceforge.net.
9
2. Chai, J.-D.; Head-Gordon, M., Long-range corrected hybrid density functionals with damped
atom-atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615-6620.
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